1. Field of the Invention
The present invention relates to a belt device, a belt deviation detecting device, and an image forming apparatus.
2. Description of the Related Art
Among image forming apparatuses such as copiers and printers is known a tandem color image forming apparatus including an intermediate transfer belt (belt member) as disclosed in, for example, Japanese Patent Application Laid-open Nos. 2006-343629 and 2001-83840.
A tandem color image forming apparatus includes an intermediate transfer belt and four photosensitive drums (image carriers) that are opposed to the intermediate transfer belt. Black (K), yellow (Y), magenta (M), and cyan (C) toner images are formed on the photosensitive drums, respectively. The K, Y, M, and C toner images are transferred onto the intermediate transfer belt in a superimposed manner to obtain a superimposed toner image. The superimposed toner image on the intermediate transfer belt is then transferred as a color image onto a recording medium.
For example, Japanese Patent Application Laid-open Nos. 2006-343629 and 2001-83840 disclose conventional technologies related to such an image forming apparatus for detecting deviation of a belt member in a direction of a width of the belt member (hereinafter, “belt width direction”) and correcting the deviation based on a result of the detection. The conventional technologies are made to reduce inconveniences that, for example, the belt member is twisted or deviates in the belt width direction to the extent that it comes in contact with a different unit and thus is damaged.
Specifically, the conventional image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-343629 includes a sensor that detects an amount of swaying of a swaying member that is in contact with an edge portion of an intermediate transfer belt (endless belt) as a belt member and that sways along with deviation of the intermediate transfer belt. Based on a result of the detection, a correcting unit (deviation correcting roller) corrects the deviation (twist) of the intermediate transfer belt.
The above conventional image forming apparatus cannot accurately detect deviation of the belt member in the belt width direction when the belt member deviates in a direction perpendicular to the belt width direction that is not a direction in which the belt member rotates (moves) (hereinafter, “belt rotation direction”). In other words, in addition to a deviation component in the belt width direction that is supposed to be detected, the detecting unit sometimes detects a deviation component in a direction other than the belt width direction and the belt rotation direction. This decreases the accuracy in correcting the deviation of the belt member based on the result of the detection by the detecting unit.
The belt member deviates in the direction other than the belt width direction and the belt rotation direction because the belt member supported by and stretched over a plurality of rollers moves while waving in the direction perpendicular to the belt width direction. Such waving of the belt member frequently occurs particularly in a high-speed image forming apparatus in which a belt member moves at high speed (i.e., an image forming apparatus having a high process linear speed).
The belt member is not limited to an intermediate transfer belt and can be a transfer-conveyer belt or a photosensitive belt deviation of which is detected and corrected.
In addition, in the conventional image forming apparatus, the deviation of the belt member in the belt width direction may not be detected with high accuracy.
Specifically, the belt member of the conventional image forming apparatus is in contact with the swaying member with a large area. Thus, when the belt member deviates (twists) in the direction perpendicular to the belt width direction (not the belt rotation direction but the vertical direction), the swaying member may sway along with the deviation in the vertical direction, which may results in inaccurate detection of the deviation of the belt member in the belt width direction. Moreover, variation in accuracy in attaching the swaying member to the belt member (an angle at which the swaying member is attached to the belt member) tends to cause variation in the result of detecting the deviation of the belt member in the belt width direction. Furthermore, the swaying member and the belt member are frayed or worn after moving for a long period while in contact with each other. This leads to a chronological change in the result of detection of the deviation of the belt member in the belt width direction.
The above inconveniences cannot be ignored especially in a high-speed image forming apparatus.